1. Field of the Invention
This invention relates broadly to the hydrocarbon industry. More particularly, this invention relates to neutron logging tools useful in the exploration of geological formation. The invention has particular application to accelerator-based neutron porosity tools although it is not limited thereto.
2. State of the Art
Conventional neutron porosity tools such as the CNT (a trademark of Schlumberger) tool or Compensated Neutron Tool, detect neutrons emitted by a radioisotope-based neutron source after they have traveled through the formation under investigation. The most common neutron source consists of radioactive americium (241Am) combined with beryllium, or AmBe. The alpha particles emitted by 241Am react with beryllium nuclei and emit neutrons of a broad spectrum of energies which span a range of about 11 MeV as seen in FIG. 1. Because of its broad energy spectrum, several different transport processes are involved as neutrons from the AmBe source scatter through the formation and lose energy. As seen in FIG. 27 for neutron energies above about 6 MeV, inelastic scattering is the dominant energy loss mechanism. For neutron energies below about 6 MeV, elastic scattering from hydrogen is the dominant energy loss mechanism. Thus, neutrons emanating from the AmBe source will be subjected to both inelastic scattering and elastic scattering.
Other neutron sources have emission spectra different from the AmBe source. For example, as seen in FIG. 1, a radioactive californium source has a peak at about 1 MeV, and emits few neutrons above 6 MeV. Accelerator-based neutron generators such as DD (deuterium source-deuterium target) and DT (mixed deuterium/tritium source and target, which is typically provided with 50% deuterium and 50% tritium, and the overwhelming interaction is deuterium-tritium) typically emit neutrons at a single energy. For example, the DD neutron generator typically emits 2.45 MeV neutrons, while the DT neutron generator typically emits 14 MeV neutrons. DT sources surrounded by a inch shell of tungsten (1 inch W Moderated DT) have spectra that show neutron emissions in a 0-2 MeV window and a 13-14 MeV window (as seen in FIG. 1). As a result, the neutrons from californium and DD neutron sources are generally subject to primarily elastic scattering, while the neutrons from DT and 1 inch W Moderated DT are subject to both inelastic and elastic scattering. It is noted that the neutrons from DT are subject to elastic scattering after the energies of the neutrons are first reduced by inelastic scattering.
For certain measurements such as formation porosity, tools which utilize an AmBe source and two detectors have been dominant in the marketplace. However, because of security issues associated with radioactive materials, it is no longer desirable to use AmBe as a source material.